A percussion implement of the type here contemplated has been disclosed, for example, in commonly owned French Pat. No. 1,431,835 and patent of addition No. 2,045,289. Reference in this connection may also be made to my copending application Ser. No. 396,806, filed July 9, 1982.
Such a percussion implement generally has a body designed as a power cylinder with a bore which in normal use is more or less vertical and has an open lower end forming a seat for a working tool, e.g. a drill or a chisel. The tool is impacted by a massive piston or ram reciprocable in the cylinder bore, this ram having a piston head received in an enlargement of the bore which is either permanently connected or intermittently connectable to a source of hydraulic liquid under pressure. The high-pressure liquid acts upon the annular underside of the piston head so as to drive the ram upward when the space thereabove, constituting a control chamber, is connected to the low-pressure side of the source; when that control chamber communicates with the high-pressure side, the resulting pressure differential drives the ram down into contact with the tool.
The reciprocation of the ram causes the leakage of hydraulic fluid (referred to hereinafter as oil) from the enlarged pressurized part of the bore toward the lower end of the arm through a surrounding annular clearance. It has already been proposed to surround that lower end with an oil seal and to dispose a drainage chamber, formed by an annular recess of the cylinder bore, between the oil seal and the enlargement connected or connectable to high pressure. The drainage chamber has an outlet through which leakage oil is returned to the low-pressure side of the source, usually by way of flexible tubing.
When the ram strikes the tool, a backward-propagating pressure wave is generated which is partly absorbed by the cylinder body and transmitted thereby to the drainage chamber. The shock of the impact momentarily presses on the oil in that chamber while the ram tends to broaden as a result of the blow, thus constricting the leakage path and contributing to the pressurization of the chamber. That brings about a sharp rise in oil pressure which is not immediately absorbed by the intake side of the source and is followed by a sudden drop or cavitation; this phenomenon, recurring with every blow of the ram, is rather detrimental to the oil seal and to the connection from the drainage chamber to the source.